In recent years there has been a great deal of interest in the transmission of various types of information television signals via optical fiber. Presently, most CAW systems for distributing television signals operate by modulating the video, audio, and other information for each television channel onto a respective radio frequency carrier signal. Each of these carrier signals typically has a bandwidth of 6 MHz (4.5 MHz of information and a 1.5 MHz guard band). A plurality of these signals covering a broadband of radio frequencies (e.g., in the range of 54-550 MHz) are distributed via networks comprising 75 ohm coaxial cables and appropriate signal amplifiers and taps.
Optical fibers intrinsically have more information carrying capacity than do the coaxial cables which are used in present CATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals. Consequently, optical fibers are capable of spanning longer distances between signals regenerators or amplifiers than are coaxial cables. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by electrical shorting or radio frequency pickup. Finally, optical fiber is immune to ambient electromagnetic interference ("EMI") and generates no EMI of its own.
A number of means are available for transmitting television signals and or other types of information over optical fibers or other optical transmission media. For example, the 6 MHz baseband television signal may be converted to digital form. This digital information may be used to modulate a light signal and transmitted via an optical link. Transmission of such a digitized 6 MHz video signal requires a digital data transmission rate of at least 45 megabits per second. High definition video ("HDTV") may require a digital data transmission rate of up to 145 megabits per second. Moreover, encoders and decoders for converting analog television signals to digital form and for reconverting these digital signals to analog form for viewing on a conventional television set are quite expensive. Consequently, analog transmission of television signals by optical means is, potentially, much more economical than digital transmission of such signals.
One such means of analog transmission is to use a baseband television signal to frequency modulate a radio frequency carrier. This modulated radio frequency carrier is in turn used to modulate an optical signal. Such frequency modulation is less susceptible to noise than is amplitude modulation, but it requires more bandwidth for each television channel transmitted than is required by amplitude modulation. Thus, the number of television channels which can be carried by each optical transmission link (e.g., each optical fiber) in an FM-based system may be somewhat limited. Moreover, since the standard NTSC format for video calls for amplitude modulation of the video carrier, means for converting FM signals to NTSC AM format are required either at the television set or at the point at which the fiber transmission trunk is connected to a coaxial cable distribution network. The need for such FM to NTSC AM conversion increases the cost of the system.
In view of the above, a system in which the video baseband signal amplitude modulates a radio frequency carrier signal which in turn amplitude modulates an optical signal is preferable to other systems from the standpoint of cost and simplicity. However, several phenomena limit the number of radio frequency channels which can be carried by present day optical links where the intensity of light signals is amplitude modulated. A first of these phenomena is a limitation of the amount of radio frequency energy which may be supplied as a modulating signal to a laser or other light generating device before various types of distortions are generated by the light generating device. This power limitation relates to the sum of the radio frequency power contributions of each radio frequency channel. Thus, if it were desired to transmit 80 radio frequency channels over a single optical link, each of these channels could be powered with only half of the power which would be available if only 40 channels were to be transmitted by the link. Such a limitation on the power of each radio frequency carrier brings each of these carriers closer to the white noise level of the system and, thus, adversely affects the signal to noise ratio of the system. Decreasing the number of channels carried by each optical link in order to improve the signal to noise ratio increases the number of lasers which must be used and the overall complexity and cost of the system. On the other hand, trying to increase the amount of radio frequency power supplied to the laser beyond certain limits causes the laser to produce several types of distortion which are discussed below.
When the modulating signal supplied to a laser causes the laser to be driven into a nonlinear portion of its input-signal-to-light-output characteristic, harmonic distortion may be produced. The products of this type of distortion are signals which are integer multiples of the "primary" frequency. The second harmonic of 54 MHz is, for example, 108 MHz. Thus, if the bandwidth accommodated by a system is such that there are channels at both 54 MHz and 108 MHz, second harmonics of the 54 MHz channel will interfere with the signals on the 108 MHz channel.
Intermodulation distortion is of particular concern in amplitude modulated systems. Such distortion results in distortion products having frequencies which are the sum or difference between two other frequencies. The sum and difference of two primary frequencies are called second order distortion products and are particularly troublesome. For example, a video channel at 150 MHz and another video channel at 204 MHz may produce a second order distortion product at 54 MHz (the difference frequency) and at 354 MHz (the sum frequency). Third order distortion products are produced by the mixing of a primary frequency and a second order distortion product producing third order distortion products equal to the sum and difference between the primary frequency and the second order distortion product. Third order products may also be generated by the mixing of three frequencies or by the third harmonic generation of a primary frequency.
Several methods have been proposed to alleviate the problems caused by harmonic distortion and intermodulation distortion in amplitude modulated optical links. One such method is described in U.S. Pat. No. 5,153,763 to Pidgeon, which is assigned to the Assignee of the present application. In accordance with the method described in the above application, distortion of a broadband CATV signal is reduced by translating the signal to a higher frequency range at which the bandwidth of the signal constitutes less than an octave. This translated broadband signal is applied as an radio frequency modulating signal to an optical signal generating device (e.g., a laser or laser diode) and transmitted over an optical link (e.g., a fiber optical path) to an optical receiver and demodulator at a remote location. After demodulation radio frequency signals outside the band of interest (e.g., harmonics and intermodulation distortion products) are filtered out by a bandpass filter having a passband covering only the frequencies of interest. The passband is then reconverted to the original frequency range (e.g., 54 to 550 MHz) and is available for distribution to subscribers of conventional CATV components.
One deficiency of the above method is that the processes of "up-conversion" and subsequent "down-conversion" may themselves introduce noise and/or distortion into the system.